Editing Interstellar travel (section)

==== Nuclear fusion rockets ====
[[Fusion rocket]] starships, powered by [[nuclear fusion]] reactions, should conceivably be able to reach speeds of the order of 10% of that of light, based on energy considerations alone. In theory, a large number of stages could push a vehicle arbitrarily close to the speed of light.<ref name="L.D. Jaffe 1963, pp. 49-58" /> These would "burn" such light element fuels as deuterium, tritium, <sup>3</sup>He, <sup>11</sup>B, and <sup>7</sup>Li. Because fusion yields about 0.3–0.9% of the mass of the nuclear fuel as released energy, it is energetically more favorable than fission, which releases <0.1% of the fuel's mass-energy. The maximum exhaust velocities potentially energetically available are correspondingly higher than for fission, typically 4–10% of the speed of light. However, the most easily achievable fusion reactions release a large fraction of their energy as high-energy neutrons, which are a significant source of energy loss. Thus, although these concepts seem to offer the best (nearest-term) prospects for travel to the nearest stars within a (long) human lifetime, they still involve massive technological and engineering difficulties, which may turn out to be intractable for decades or centuries.

[[File:Daedalus ship.png|thumb|Daedalus interstellar probe]]Early studies include [[Project Daedalus]], performed by the [[British Interplanetary Society]] in 1973–1978, and [[Project Longshot]], a student project sponsored by [[NASA]] and the [[US Naval Academy]], completed in 1988. Another fairly detailed vehicle system, "Discovery II",<ref>[https://web.archive.org/web/20110610051632/http://gltrs.grc.nasa.gov/reports/2005/TM-2005-213559.pdf PDF] C. R. Williams et al., 'Realizing "2001: A Space Odyssey": Piloted Spherical Torus Nuclear Fusion Propulsion', 2001, 52 pages, NASA Glenn Research Center</ref> designed and optimized for crewed Solar System exploration, based on the D<sup>3</sup>He reaction but using hydrogen as reaction mass, has been described by a team from NASA's [[Glenn Research Center]]. It achieves characteristic velocities of >300&nbsp;km/s with an acceleration of ~1.7•10<sup>−3</sup> ''g'', with a ship initial mass of ~1700 metric tons, and payload fraction above 10%. Although these are still far short of the requirements for interstellar travel on human timescales, the study seems to represent a reasonable benchmark towards what may be approachable within several decades, which is not impossibly beyond the current state-of-the-art. Based on the concept's 2.2% [[burnup]] fraction it could achieve a pure fusion product exhaust velocity of ~3,000&nbsp;km/s.